1. Technical Field
The present invention relates to earth boring bits, and more particularly to those having rotatable cutters, also known as cones.
2. Description of Related Art
Earth boring bits with rolling element cutters have bearings employing either rollers as the load carrying element or with a journal as the load carrying element. The use of a sealing means in such rock bit bearings has dramatically increased bearing life in the past fifty years.
Early seals for rock bits were designed with a metallic Belleville spring clad with an elastomer, usually nitrile rubber (NBR). The metallic spring provided the energizing force for the sealing surface, and the rubber coating sealed against the metal surface of the head and cone and provided a seal on relatively rough surfaces because the compliant behavior of the rubber coating filled in the microscopic asperities on the sealing surface. Belleville seals of this type were employed mainly in rock bits with roller bearings. The seal would fail due to wear of the elastomer after a relatively short number of hours in operation, resulting in loss of the lubricant contained within the bearing cavity. The bit would continue to function for some period of time utilizing the roller bearings without benefit of the lubricant.
A significant advancement in rock bit seals came when o-ring type seals were introduced. These seals were composed of nitrile rubber and were circular in cross section. The seal was fit into a radial gland formed by cylindrical surfaces between the head and cone bearings, and the annulus formed was smaller than the original dimension as measured as the cross section of the seal. The o-ring seal was then radially squeezed between the cylindrical surfaces.
To minimize sliding friction and the resultant heat generation and abrasive wear, rotating O-rings are typically provided with a minimal amount of radial compression. However, reciprocating (Belleville) seals must have a much larger radial compression to exclude contamination from the sealing zone during axial sliding (typically about twice the compression). The rock bit seal must both exclude contamination during relative head/cone axial motion and minimize abrasive wear during rotation.
Reference is now made to FIG. 1 which illustrates a prior art configuration for an earth boring bit. FIG. 2 illustrates a close-up view of the prior art configuration focusing on the area of a sealing system 2 associated with a rotating cone 4 installed on a shaft 6 of a bit head 8. An o-ring seal 10 is inserted into a seal gland 12 and squeezed between a cone sealing surface 14 and a head sealing surface 16.
Reference is now made to FIG. 3 which illustrates a prior art configuration for an earth boring bit. FIG. 4 illustrates a close-up view of the prior art configuration focusing on the area of a sealing system 22 associated with a rotating cone 24 installed on a shaft 26 of a bit head 28. A first ring 30 is press-fit into a gland 32 formed in the cone 24. The first ring 30 presents a first metal seal face 34. A second ring 36 is also placed in the gland 32. The second ring 36 presents a second metal seal face 38. An energizing structure 40 is also placed in the gland 32 and configured to apply a combination of axial and radial force against a back surface 42 of the second ring 36 so as to urge the second metal seal face 38 into contact with the first metal seal face 34. The structure shown in FIG. 4 illustrates the well-known single energizer type of metal faced sealing system.
In all configurations of metal faced sealing structures, the sealing system 22 must be provided with sufficient force through the energizing structure 40 to maintain sufficient sealing contact (between the second metal seal face 38 and first metal seal face 34) and further to overcome any pressure differential between internal and external zones. Pressure differentials between those zones fluctuate as the cone is contorted on the bearing during operation. This phenomenon is known in the art as “cone pumping.” Cone pumping throws an internal pressure surge at the metal faced bearing seal which can lead to catastrophic failure of the seal over time. In addition, changes in depth while the bit is in use can cause fluctuations in pressure between the internal pressure and the external pressure. Conversely, application of too much force on the seal by the energizing structure 40 can cause difficulties in assembling the cone to the bearing and may result in accelerated wear of the first and second rings 30 and 36. It is important that the metal seal faces 34 and 38 are flat, and so a lapping of the surfaces is often provided (in the light band range).
A significant challenge with the single energizer type of metal faced sealing system shown in FIG. 4 is that the press fitting of the first ring 30 in the cone gland 32 may deform the first ring and produce a “waviness” in the first metal seal face 34. The second ring 36 with second metal seal face 38 must overcome this surface waviness through the force applied by the energizing structure 40 so as to maintain the desired sealing contact (otherwise the seal will leak).
An additional challenge with the single energizer type of metal faced sealing system shown in FIG. 4 is that the elastomeric energizing structure 40 is offset so as to apply force to the second ring 36 not only in the preferred axial direction but also in the radial direction. The sealing force is accordingly dissipated by the wasted force component applied in the radial direction. The radially applied force component further introduces a torque on the second ring 36 which reduces (i.e., narrows) the radial width of the effective sealing surface where the metal seal faces 34 and 38 make sealing surface contact. The reduction in width arises because the introduced torque causes a distortion of the seal ring producing an out-of-flatness surface condition on the sealing face of the seal ring.
Yet another challenge with the single energizer type of metal faced sealing system shown in FIG. 4 is that the metal seal faces 34 and 38 become unloaded as a result of an increase in grease pressure. For example, rock bit bearings may operate with an internal pressure greater than the environment. As a result, grease leakage may occur.
Notwithstanding the foregoing challenges, metal faced sealing systems are often used in roller cone drill bits which operate at higher RPM drilling applications because the metal seal faces 34 and 38 resist wear and consequently exhibit longer operating life than a standard O-ring type sealing system like that shown in FIGS. 1 and 2.
The foregoing challenges remain an issue and thus a need exists in the art for an improved metal faced sealing system for use in rock bits.